JPS6157874B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to coating compositions, and more particularly to water-dispersible coatings for metal containers. Food and beverage cans have traditionally been made of iron, tin,
Containers made of materials such as aluminum have been used. In order to protect the contents packed in these containers from contamination by the container, i.e. metal, the interior of the container is coated with an organic paint that is essentially inert to the contents and provides an effective barrier between the container and the contents. It needs to be covered with. Currently, phenolic epoxy resins dissolved in organic solvents, epoxy
Organic paints such as urea resins are used as protective coatings. These resins exhibit excellent adhesion to metal materials, act as an effective barrier between the container and the contents, and have high barrier properties against water and other contents. In this sense, it is suitable as an inner surface coating for metal containers (hereinafter referred to as can inner surface coating). However, in reality, various external forces are applied during the can manufacturing process, so the resin that serves as the base of the paint must be highly flexible in addition to the above-mentioned properties. When a thermosetting resin such as a phenol epoxy resin or an epoxy-urea resin is heated on a metal plate, its properties such as barrier properties, adhesion to contents, and flexibility of the resin change greatly depending on the heating conditions. It is extremely difficult to maintain all of these properties at appropriate levels simply by adjusting the heat treatment conditions. In fact, during the can-making process, when forming processes such as press-forming can lids, flanging can bodies, and tightening windings are performed, damage to the paint film and partial peeling are observed. in this case,
The workability of the coating film is not sufficient, and even the slightest damage can cause areas with poor adhesion, and corrosion of exposed metal progresses. As a result, the original function of the food packaging material, which is to protect the food over a long period of time without impairing the flavor and freshness of the contents, is not fully achieved. Therefore, in the actual can manufacturing process, the above-mentioned phenol-epoxy resin or epoxy-urea resin is applied to the metal as an undercoat, and then another additive is applied over it to form a single coating film of these thermosetting resins. The current situation is that they are making up for their shortcomings. in this case,
Resins used for surface coating include vinyl chloride-vinyl acetate copolymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer, and the like. Furthermore, although a single coating film of these PVC resins has excellent barrier properties and processability, it has the disadvantage of poor adhesion to metal materials. In addition, the inner surface of these cans contains organic solvents that become volatile components that evaporate from the coating composition during curing, so in consideration of air pollution, the organic solvent gas generated during curing is burned through an afterburner. Although it burns a large amount of solvent,
Of course, this is not a good idea in terms of energy consumption. In this sense, the development of a water-dispersible can interior paint is strongly desired. As mentioned above, regardless of whether thermosetting resin or thermoplastic resin is used, the width required for can inner coatings is required in terms of barrier properties, adhesion, processability, sanitary properties, flavor, and boiling resistance against the contents. It can be said that at present, there are almost no resins with a wide range of properties. In this sense, if this resin is suitable as a paint for the inside of a can and can be made into a water-dispersible type, two major problems will be solved at the same time, and its merits will be enormous. In general, if a thermoplastic polyester resin consisting of a combination of aromatic dibasic acid, aliphatic dibasic acid, and diol components is used as a paint for the inside of a can, it will improve the processability of metal materials, the hygiene properties such as the extraction resistance of the paint film, and the paint quality. It is known to have a wide range of suitability for combinations of workability and wide applicability to various contents. However, when considering the properties of thermoplastic polyester resins from the viewpoint of composition, among aromatic dibasic acids, for example, as the amount of terephthalic acid increases, properties such as boiling resistance improve, but conversely, the solubility or dispersibility in solvents improves. , film-forming properties, adhesion to metals,
Properties such as processability deteriorate. The same is true for diol components. For example, if a crystalline component such as ethylene glycol or 1,4-butanediol is used as a diol component, the resulting coating film will have properties such as boiling resistance, but conversely, it will have poor solubility or dispersion in the solvent. Properties such as hardness, film-forming properties, and processability deteriorate. Even when other diols are used, they often exhibit contradictory properties, such as boiling resistance and solubility or dispersibility in solvents.
In reality, problems are solved by appropriately combining these raw materials or by using aromatic dibasic acids together with a certain amount of aliphatic dibasic acids or polyether diols. However, when combining these dibasic acid and diol components to obtain a resin that maintains the desired properties in a well-balanced manner, the monomers that will become the thermoplastic polyester resin component must be selected very strictly. Not only is this inconvenient, but the use of aliphatic dibasic acids, polyether-type diols, etc. can improve the boiling resistance, film formation, solubility in solvents, dispersibility, processability, etc. of the coating film. Trying to maintain a good balance with nature is actually quite difficult. The present inventors have focused on the fact that thermoplastic polyester resin has extremely good properties such as hygiene, adhesion to metal containers, and processability, and in order to use this resin more advantageously, we have developed We carefully considered the interior paint. As a result, as a method to take advantage of the advantageous properties that arise because thermoplastic polyester resins are linear and have a high molecular weight, it is possible to react the terminal hydroxyl groups of thermoplastic polyester resins with polymerizable unsaturated carboxylic acids or acid anhydrides thereof. A stable water-dispersed polyester copolymer was obtained by carrying out a radical polymerization reaction using the polymerizable unsaturated bonds. The obtained copolymer was confirmed to be free of extractables and has good hygiene properties, and also showed good results in terms of boiling resistance, adhesion to metal containers, processability, chemical resistance, etc. The present invention was achieved by confirming this. The present invention involves an esterification reaction (A) of a thermoplastic polyester resin having a terminal hydroxyl group and a polymerizable unsaturated carboxylic acid or its acid anhydride, and then a (meth)acrylic acid derivative and, if necessary, a polymerizable inorganic acid. A polyester copolymer obtained by radical polymerization reaction (B) of a mixture of saturated carboxylic acids or a mixture of styrene added thereto, neutralized with a basic substance, and dispersed in water. This is a water-dispersible paint for metal cans. If the water-dispersed paint of the present invention is used as a paint for the inside of a can, not only can the characteristics resulting from the high molecular weight of the thermoplastic polyester resin itself be utilized, but if the monomers used in the reaction are appropriately selected, the resin can be It is also possible to synthesize paints with low viscosity at high concentrations. In addition, in order to impart barrier properties and contents protection functions to the coating film as a can interior paint,
A certain film thickness is required. If a paint with a low resin concentration is used for this purpose, problems such as paint sagging and thick coating on the bottom of the can will occur. At the same time, this may cause brittleness to occur at the bottom of the can. Preparing a paint with a high resin concentration and low viscosity like the water-dispersed paint of the present invention not only makes it easier to obtain the desired coating, but also improves operational stability and air pollution, which are the features of the water-dispersed paint. , which is extremely advantageous in that it is highly economical. The thermoplastic polyester resin used in the present invention has good properties as a resin film itself such as boiling resistance, but resins with poor properties such as dispersibility in water and stability as a paint, that is, resins with a large molecular weight and crystalline Thermoplastic polyester resins with high properties and few functional groups are preferred. On the other hand, some thermoplastic polyester resins are known to dissolve or disperse in various solvents at room temperature, but these types of resins often have poor properties as a coating film, such as boiling resistance. One of the reasons for this is the low crystallinity of this resin. One way to improve this drawback is to introduce functional groups into the resin so that it is possible to take measures to improve the physical properties. According to the present invention, carboxyl groups, hydroxyl groups, etc. are introduced into the resin by appropriately selecting a compound having a polymerizable unsaturated bond that can be reacted even if the thermoplastic polyester resin itself is not resistant. By self-crosslinking or reaction with various resins such as melamine resins, epoxy resins, phenolic resins, and urea resins, coating films with excellent physical properties can be obtained. Therefore, in the present invention, the thermoplastic polyester resin is not limited to only high molecular weight and highly crystalline resins. Examples of aromatic dibasic acids that can be used in the synthesis of the thermoplastic polyester resin used in the present invention include dibasic acids such as terephthalic acid, isophthalic acid, and orthophthalic acid. In particular, as the content of terephthalic acid increases, the boiling resistance of the coating film becomes better, but the adhesion to metal becomes poorer. As the content of isophthalic acid and orthophthalic acid increases, adhesion to metals, solubility or dispersibility in solvents, etc. become better, but processability such as bending is not necessarily good. Aliphatic dibasic acids can also be used to balance various properties. For example, dibasic acids such as adipic acid, sebacic acid, malonic acid, and succinic acid can be mentioned, but in general, the use of aliphatic dibasic acids is effective in softening the resin, but on the contrary, the properties of the coating film are It comes with the disadvantage of being lower.
Therefore, the types and amounts of aromatic dibasic acids and aliphatic dibasic acids must be appropriately combined.
On the other hand, as the diol, for example, one or two types from ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 1,6-hexanediol, etc. obtained from the above combinations.
It is also possible to use polyether glycols, but their type and amount are determined by the combination with other diols. The polymerizable unsaturated carboxylic acids or their acid anhydrides used in the reaction (A) of the present invention include acrylic acid,
Examples include monobasic acids such as methacrylic acid, dibasic acids such as itaconic acid, maleic acid, succinic acid, and fumaric acid, and acid anhydrides such as itaconic anhydride and maleic anhydride. In reaction (A), a thermoplastic polyester resin having a terminal hydroxyl group is mixed with a solvent such as toluene, xylene, chloroform, cellosolve acetate, methyl ethyl ketone, dioxane, tetrahydrofuran, cyclohexanone, etc. alone or in combination.
The mixture is heated and dissolved at a concentration of 60% by weight or more, and the hydroxyl groups of the thermoplastic polyester resin and the unsaturated carboxylic acid or their acid anhydrides are reacted at a temperature of 140 to 160° C. for 1 to 5 hours under a nitrogen stream. The reaction is allowed to proceed by distilling off water in a conventional manner, but at this time it is possible to use a small amount of an ordinary esterification catalyst and polymerization inhibitor in order to advance the esterification reaction. The end point of esterification is confirmed by measuring the hydroxyl value of the resin. Reaction (B) of the present invention is similar to normal radical polymerization using the product (esterified product) obtained by reaction (A) and a compound having a polymerizable unsaturated bond (hereinafter referred to as a polymerizable unsaturated compound). It can be done by method. Incidentally, polymerizable unsaturated carboxylic acids or acid anhydrides thereof can be used as reaction components in both reactions (A) and (B). Furthermore, in order to obtain a stable water-dispersed paint, it is necessary to accurately limit the amount of the polymerizable unsaturated carboxylic acid or acid anhydride contained in the composition used as the polymerizable unsaturated compound. The amount of the polymerizable unsaturated carboxylic acid or the polymerizable unsaturated compound containing these acid anhydrides used in reaction (A) and reaction (B) is in the range of 5 to 50% by weight based on the polyester copolymer. Preferably,
Furthermore, 10 to 30% by weight is suitable. If it is less than 5% by weight, it is difficult to obtain a stable dispersion, and if it is more than 50% by weight, the coating film becomes cloudy. The amount of polymerizable unsaturated carboxylic acids or their acid anhydrides used in the polymerizable unsaturated compounds is 1.0 including both reaction (A) and reaction (B).
A range of ~15% by weight is desirable; if it is less than 1.0% by weight, it is difficult to obtain a stable aqueous dispersion, and if it is more than 15% by weight, the proportion of homopolymerization increases, resulting in the disadvantage that it is difficult to obtain a high-quality coating film. Of course, it is possible to a certain extent to balance the properties of the water-dispersed polyester copolymer produced by appropriately selecting the type of polymerizable unsaturated compound used in the reaction; The most important factor determining the properties of the resin is the content of the polymerizable unsaturated compound used in the reaction with the thermoplastic polyester resin. Polymerizable unsaturated compounds that can be used in reaction (B) of the present invention include acrylic acid and methyl acrylate,
Acrylic acid derivatives such as ethyl acrylate, 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, methacrylic acid derivatives such as methacrylic acid and methyl methacrylate, isobutyl methacrylate, lauryl methacrylate, glycidyl methacrylate, maleic anhydride, It can be selected from acid anhydrides such as itaconic anhydride, styrene, vinyl acetate, acrylonitrile, 1,2-polybutadiene, 1,4-polybutadiene, etc. alone or in combination. In reaction (B), a radical polymerization catalyst such as azobisisobutyronitrile, benzoyl peroxide, dicumyl peroxide, etc. is usually used. The polyester copolymer according to the present invention can be made into an aqueous dispersion by neutralizing it with a basic substance. The basic substance is preferably a substance that volatilizes during coating film formation, and ammonia aqueous solution, amine, etc. are suitable. Examples of the amine include diethylamine, dimethylaminoethanol, diethylaminoethanol, and diethylaminopropanol. As the reaction for obtaining the polyester copolymer according to the present invention, it has been mainly described that the reaction (A) is carried out and then the reaction (B) is carried out, but the reactions (A) and (B) are carried out simultaneously. You may do so. The coating material of the present invention may contain commonly used additives. The water-dispersed paint of the present invention has a milky white appearance,
It has a nonvolatile content of 10 to 30% by weight, and is an extremely safe paint that does not cause any resin separation or sedimentation even when stored at 40°C. In the polyester copolymer of the present invention, it is thought that the polymerizable unsaturated compound used in the reaction is oriented and stably dispersed in water while being coated with a highly crystalline thermoplastic polyester resin. Even if the thermoplastic polyester resin itself is not dissolved or dispersed in water at room temperature, a stable dispersion type coating material can be obtained by the present invention. Therefore, the present invention can be particularly effectively applied when the thermoplastic polyester resin itself has good properties as a coating film, such as boiling resistance, but lacks solubility or dispersibility. In order to confirm that the water-dispersible paint of the present invention has features, the water-dispersible paint of the present invention,
A comparison was made with a blend obtained by mixing the original thermoplastic polyester resin with a resin separately synthesized using only a polymerizable unsaturated compound. the result,
The original thermoplastic polyester resin itself does not dissolve or disperse in water, and even ordinary surfactants cannot disperse the resin in water.
Blends with resins synthesized in advance using only polymerizable unsaturated compounds generally have poor stability, and often separate or settle during blending under heat or when left to stand. Even if an aqueous dispersion is obtained, when it is made into a coating film, the transparency is considerably poorer than that of the water dispersion type paint of the present invention, and the phenomenon of cloudiness is observed, which proves that the present invention is significant. confirmed. When the water-dispersible paint of the present invention is used as a can inner surface paint, it can be applied directly to the surface of the metal container material, but it can also be applied over conventional epoxy-phenol resin, epoxy-urea resin, melamine-epoxy resin, etc. It can also be used as a top coating agent. In any of these cases, film forming properties,
It has good properties such as transparency, adhesion, boiling resistance, potassium permanganate consumption, and processability. The water-dispersible paint of the present invention can be applied to metal materials or various plastics by any method such as spray painting, roll coating, knife coating, or air knife coating. A protective coating must be an effective barrier between the container and its contents, and a coating method must be able to perform its original function. In this sense, the thickness of the resin coating also plays an important role, but when considering unavoidable pinball and the like, it is practically desirable to use it in combination with the thermosetting resin coating. If the water-dispersible paint of the present invention is used as a paint for the inside of a can, not only will the characteristics of the thermoplastic polyester resin due to its high molecular weight be utilized, but it will not be able to be dissolved or dispersed in water at room temperature. Even thermoplastic polyester resins can be stably dispersed in water. Moreover, since it is a dispersion type paint, it is also possible to synthesize a paint with a high resin concentration and low viscosity. With the dispersion type paint of the present invention, it is possible to prepare a paint with high resin concentration and low viscosity.
It is very advantageous in that it is not only easy to obtain the desired coated product, but also has excellent operational safety, air pollution, and economical efficiency. Next, the present invention will be explained by examples. Example 1 A thermoplastic polyester resin was synthesized in advance as follows. dimethyl terephthalate (0.8 mol),
Dimethyl isophthalate (0.2 mol), ethylene glycol (1.5 mol) and 1,4-butadiol (1.5 mol) were mixed and heated to 180°C under a nitrogen stream.
The reaction is allowed to proceed for ~2 hours, and the amount of methanol removed is measured. After the demethanol reaction is completed, the pressure of the reaction system is reduced (1 mmHg) and the temperature is raised to 220°C in about 30 minutes. The reaction is carried out at 220°C for about 4 hours to accelerate the deglycol reaction, increasing the viscosity of the resin and producing a thermoplastic polyester resin. The molecular weight and softening point of the obtained thermoplastic polyester resin were 26,000 and 84°C, respectively. The resulting thermoplastic polyester resin was dissolved in cyclohexanone at 120° C. and applied to a tin plate using a 6 mil applicator while hot in a 5 μm coating. A transparent coating film was obtained, and no whitening or other abnormalities were observed even when subjected to a boiling test at 100°C for 30 minutes. However, the polyester resin was not dissolved or dispersed at all in single or mixed solvents such as cyclohexanone, cellosolve acetate, xylene, and methyl ethyl ketone at room temperature. Next, 3% of methacrylic acid and 0.2% by weight of P-toluenesulfonic acid were added to the obtained thermoplastic polyester resin and cellosolve acetate/xylene (1/
1) Mix in a mixed solvent to make the thermoplastic polyester resin concentration 65% by weight. Next, the reaction system was heated to 150°C.
The temperature was raised to 100%, and the mixture was allowed to react for 2 hours. After the reaction, the temperature of the reaction system was lowered to 80°C, and then 10% by weight of styrene/methacrylic acid (weight ratio 40/60) mixed monomer based on the thermoplastic polyester resin and 1% by weight based on the monomer. of benzoyl peroxide were mixed and radical polymerized for 2 hours. After the reaction is completed, increase the rotation speed of the stirrer from 200 rpm to 600 rpm, and add 1/2 equivalent of dimethylaminoethanol to the carboxylic acid component contained in the produced resin.
Add 80℃ warm water dropwise. When warm water is added dropwise while gradually lowering the temperature of the reaction system, the system gradually becomes cloudy and a stable water-dispersed polyester copolymer is finally obtained. The water/organic solvent ratio was 91/9 (weight ratio), and the polyester copolymer concentration was 21% by weight. The resulting water-dispersed paint was stored at 40°C for 14 days, but no abnormalities such as resin separation or sedimentation were observed. Further, the water-dispersed paint was applied to a tinplate plate with a film thickness of 125 μm using a 6 mil applicator, heat treated at 170° C. for 10 minutes, and the properties of the film were examined. The results are shown in Table 2. Examples 2 to 8 In the same manner as in Example 1, water-dispersible paints were prepared with the compositions shown in Table 1, and the results of testing according to each test method are shown in Table 2.

【table】

【table】

【table】

[Table] Test method Resin transparency: Visually evaluate the transparency of the coating film,
Display in stages. 5 (good) to 1 (bad). Film forming property: Presence or absence of cracks in the coating film. Adhesion: Performed using a cellophane tape peel test and expressed as adhesion rate. Workability: 0 (zero) T-bent, 500g load 30
Fall from a height of cm. Boiling resistance: Immersed in hot water at 100℃ for 30 minutes, indicating presence or absence of whitening in 5 levels. 5 (good) to 1 (bad). Potassium permanganate consumption: Extract with 1 ml of distilled water per 1 cm ² of painted plate at 100℃, and titrate using the usual method. Comparative Example 1 The thermoplastic polyester resin synthesized in Example 1 was dissolved in cyclohexanone, and 6% by weight of a nonionic surfactant was added to the resin.
Mixed for 15 minutes with a high speed mixer. After stirring, the mixture was cooled to 80°C, and an attempt was made to drop hot water at 100°C into the mixture, but the resin began to precipitate shortly after the start of dropping, and the resin could not be dispersed in water.

Claims (1)

[Claims] 1 After carrying out an esterification reaction (A) of a thermoplastic polyester resin having a terminal hydroxyl group and a polymerizable unsaturated carboxylic acid or its acid anhydride, a (meth)acrylic acid derivative and, if necessary, a polymerizable unsaturated carboxylic acid or a polyester copolymer obtained by radical polymerization reaction (B) of styrene added thereto, neutralized with a basic substance and dispersed in water. Paint for metal cans.